1. Field of the Invention
The present invention relates to a device for sticking a protective sheet on the substrate surface.
2. Description of the Prior Art
When the back face of a semiconductor wafer (also referred to simply as wafer hereinafter) is ground and polished, it is necessary to protect the front face of the substrate in order to avoid damage, and contamination caused by ground dust generated during the grinding work, grinding fluid, or the like. Moreover, the wafer after grinding and polishing has a problem in that it tends to be broken rather easily even by a slight external force because of its surface being irregular due to device pattern, in addition to its being thin and fragile.
For the purpose of protecting the surface during such a wafer processing, a method of sticking a plastic protective tape (protective sheet) made of resin on the surface is well-known as a general technology. Such a technique is disclosed in, for example, Japanese Patent Applications Laid Open, No. Hei 8-148452.
Now, the surface of a wafer in recent years has a conspicuous irregularity due to a pattern formed on the surface. For example, in a wafer with a polyimide film, the thickness of the film is 5 to 20 μm, and the regions of removal of the polyimide film extend even to the outer periphery of the wafer, in an electrode pad section formed in the outer peripheral portion of the pattern for establishing connection with the distributed wires, and in grooves (scribe lines) for chip dicing. Here, the width of the groove is about 50 to 100 μm.
However, even though the surface of the wafer is bonded with a protective surface according to the conventional technique as described in the above, the sheet cannot follow the irregularity of the surface of the semiconductor substrate, resulting in an incomplete adhesion between the protective sheet and the wafer surface.
Referring to FIG. 8, this situation will be described. FIG. 8A is a plan view of a wafer with a protective sheet stuck on, and FIG. 8B is a sectional view at the outer periphery of the wafer. As shown in FIG. 8A, the entire surface of a wafer 101 is bonded with a hatched surface protective sheet 102 by a pressure bonding technique.
When a region 103 shown in FIG. 8A is cut along the outer periphery of the wafer and enlarged, it looks as shown in FIG. 8B. A passivation film 104 composed of a polyimide film is formed on the surface of the wafer 101, and a scribe line 105 is formed in the passivation film 104. The passivation film 104 is bonded from above-by the surface protective sheet 102 consisting of an adhesion layer 106 and a protective layer 107.
However, in the scribe line 105 part having an irregularity, the wafer surface is not fully bonded with the adhesive layer 106, and a gap 108 is thus created. Because of this, during grinding and polishing of the back face of the wafer, the surface of the electrode pad is contaminated by the ground dust or grinding fluid that infiltrates through the gap 108. Presence of such a contamination leads to frequent occurrence of connection failure between the electrode pad and the distributed wires.
Under these circumstances, Japanese Patent Applications Laid Open, No. 2000-038556 discloses a means of filling in the irregularity, in which a sheet having a pressure sensitive adhesion layer that can be melted at a specified melting point and can follow the irregularity is stuck on the wafer, and the entire surface of the wafer is heated.
Since, however, the heating method adopted by the disclosure is one in which all the irregularities are filled by heating the entire surface of the wafer, the adhesive strength of the sheet is enhanced to such an extent that the possibility of cracking the wafer at peeling of the sheet is increased under the condition that the thickness of the wafer after the grinding and polishing is small. Moreover, since the electrode pad for bonding the distributed wires at assembly is formed of a metal with low resistance (mainly aluminum), its surface is oxidized by being heated, forming an insulator layer of metal oxides, which becomes the cause of conduction failure.